Vibrational spectra and structure of small ring compounds silacycolpentane

Abstract

The infrared spectra of silacyclopentane and 1,1-dideutero-1-silacyclopentane in both the gaseous and liquid states were recorded from 4000 to 33 cm −1. The Raman spectra of the liquids were also recorded and depolarization values were determined. The data rule out the planar configurati tn for the silacyclopentane molecules and were shown to be consistent with a C 2 structure. The 39 normal vibrations were assigned on the basis of their depolarization values, infrared band contours, isotopic shifts, and band intensities. The two low-frequency ring modes were observed at 264 and 101 cm −1 for the normal compound, and at 259 and 94 cm −1 for the deuterated molecule. A series of pronounced Q-branches with approximately 2 cm −1 spacing were observed on the low wave-number side of the 101 cm −1 band. A similar series with an approximate spacing of 1.7 cm −1 was observed for the deuterium compound. These series have been interpreted in terms of a relatively high barrier to pseudorotation of the five-membered ring. The calculated barriers were 3.9 and 4.2 kcal/mole for the “light” and “heavy” compounds, respectively. The spectroscopically derived parameter mq 0 2 was 14.2 × 10 −40 and 17.9 × 10 −40 g cm 2 for SiC 4H 10 and SiC 4H 8D 2, respectively. The barriers are considerably higher than one might expect from existing theories and data on torsional barriers.